In the United States there are approximately 2 billion discarded tires in dumps, and the quantity is growing by about 270 million tires per year. Destruction of this waste by burning creates an unacceptable air-pollution problem, so that recourse must normally be had to recycling.
The standard tire contains in addition to the reusable elastomer, which may be a rubber of natural or synthetic origin, textile constituents from the bias-ply belts used in the tires, and metallic constituents from the bead wire and the belt in steel-belted tires. Unfortunately these textile and metal constituents are intimately bonded with the rubber they are imbedded in. Hereinbelow the term "rubber" is intended to include elastomers whether of natural or synthetic origin.
Thus recourse can be had to a method such as described in my German patent 2,608,383 issued 5 Feb. 1981. Here the tires are comminuted and the particles thus produced are stretched and deposited on a flat surface. A smooth-surfaced roller is moved back and forth over the particles, exerting on them a pressure of from 5 t to 500 t. After a dozen or more passes, the crushing effect normally separates the rubber from the steel and textile.
My German laid open patent application 3,232,061 issued 1 Mar. 1984 describes an improvement on this system which uses a ridged support surface, and even a nonsmooth roller in some instances. The rubber particles are preheated to 100.degree. C. to 150.degree. C. so that these particles need only be roller crushed some four or five times to achieve the necessary degree of separation.
In both of these procedures the pressure is enormous, so that after processing the surfaces are substantially worn. The ridged support surface and roller must be machined to reform teeth on them. The result is that the finished product, which is not extremely pure, is fairly costly and, in fact, substantially more expensive than virgin rubber.
It has further been suggested to proceed in a so-called cold process, chilling the tires so that they become brittle enough to comminute in a standard hammer mill. When sufficiently chilled the rubber is no longer an elastomer, but instead is so embrittled that is can be broken like glass. Such a procedure is, however, quite expensive. About 0.5 kg to 1.0 kg of liquid nitrogen is consumed for each 1.0 kg of rubber embrittled, at a cost of about 0.3 to 0.8 DM/kg, for an average cost per kilogram of rubber of about 0.41 DM. The hammer mill and rotary drum use about 0.55 kWh/kg corresponding to about 0.08 DM/kg of rubber milled. Thus the processing costs are about 0.49 DM/kg of rubber, at best 0.15 DM/kg. The processing costs are therefore high, and the output of such a plant is normally quite limited.
In contrast the above-described patented processes use about 0.1 kWh/kg of rubber for comminution and about 0.25 kWh/kg for the crush-separation. As a result with these or similar processes the excess energy costs are about 0.04 DM/kg to 0.05 DM/kg of rubber.